Trentham's dark galaxy part I
Its kinda neat, and the suggestion is that what caused the interaction is a "dark galaxy" - a galaxy dominated completely by dark matter, with little or not normal matter. Essentially based on the point that we don't see anything like a normal galaxy where one ought to be if the galaxy pictured interacted with another galaxy. Could be. Could be the tip of the iceberg, in that it could hint at significant mass in dark matter concentrations of galaxy mass. Or we could be missing a perfectly ordinary normal galaxy somewhere near this field. Or maybe something else did the damage, the long running alternative scenario is "cannonballing" by a supermassive black hole (in excess of 100 million solar masses) which would have been ejected from some other galaxy (possibly far away - maybe millions of light years away). The ejection would occur either during a three-body interaction between supermassive black holes after successive mergers, or from the gravitational radiation reaction of a coalescence of two unequal mass supermassive black holes.
Gratuitous cat blogging
Do cats have a quantitative reasoning capability? One of our cats (Thor) was planning a daring jump to a window, and spent a lot of time staring at the assorted toys and junk on the window sill, before picking his spot and jumping. Conversely, our very proper female cat seems to be able to count "crunchies" as they are poured into a bowl, and gets very indignant if Thor gets more than she does...